Humans spend a significant portion of their lives indoors, making indoor air quality crucial for health. Indoor concentrations of gaseous compounds and particles often exceed outdoor levels due to activities like cleaning and cooking. Hypochlorite bleach, while effective as a disinfectant, releases hazardous chlorinated compounds such as hypochlorous acid (HOCl) and chloramines upon use. These compounds pose respiratory risks, potentially exacerbating conditions like asthma. Traditionally, indoor air has been modeled as well-mixed, with single-point measurements representing entire rooms. This simplification neglects the potential for spatial heterogeneity, particularly for reactive and short-lived species. This study aims to investigate the spatial and temporal variability of indoor air pollutants, particularly those generated by bleach cleaning, by combining multiple modeling approaches with extensive experimental data from the HOMEChem campaign.

Previous research often assumed well-mixed indoor environments, utilizing single-point measurements and simple box models. While computational fluid dynamics (CFD) has been used to model airflow, indoor chemistry models generally employed box models with deposition velocities, neglecting the heterogeneity of reactive species. Studies highlighting the impact of bleach cleaning on indoor air quality have documented the release of various chlorinated compounds and their potential health effects. However, a comprehensive understanding of the spatial and temporal distribution of these compounds within indoor environments remained lacking. This gap in knowledge motivated the current study to move beyond simplistic modeling assumptions and investigate the complex interplay of chemical reactions, surface interactions, and airflow in shaping indoor air pollutant distributions.

This study integrated multiple modeling approaches with experimental data from the HOMEChem campaign. A multiphase kinetic model simulated the formation and loss of bleach products, considering air exchange, gas-phase reactions, photolysis, wall loss, heterogeneous reactions, and aqueous reactions. The INDCM (INdoor Detailed Chemical Model) was used to quantify radical production rates. A CFD model, informed by the kinetic and INDCM models, resolved spatial heterogeneity. The HOMEChem campaign provided experimental data from various instruments, including a time-of-flight chemical ionization mass spectrometer (TOF-CIMS), a cavity ring-down spectrometer, and a laser-induced fluorescence instrument (LIF-FAGE), to measure various gas-phase species and OH radicals. The models accounted for outdoor-indoor air exchange, gas-phase and multiphase reactions, photolysis, wall loss, heterogeneous reactions on indoor surfaces and particles, and aqueous reactions in bleach. The CFD model incorporated key inputs from the detailed models, resolving spatial heterogeneity in the room. The study considered the production rates and reactivity of OH radicals from the INDCM and the concentrations of various bleach products from the multiphase kinetic model. The spatial distribution of pollutants was analyzed through horizontal and vertical maps. Finally, temporal and spatial scales of various indoor species were estimated by considering air exchange rates, reaction rates, photolysis, and surface deposition.

The integrated modeling approach successfully reproduced the temporal evolution of OH radicals and bleach cleaning products (HOCl, NCl3, NH3). The study revealed significant spatial and vertical concentration gradients for these compounds. High concentrations of OH radicals were confined to sunlit zones near windows, while bleach products exhibited higher concentrations near the cleaning surface and showed vertical gradients. The CFD model accurately captured the dynamic concentration changes at different sampling points. The observed enhancement of OH radicals during bleach cleaning was primarily attributed to Cl2 photolysis, initiating a cascade of reactions leading to OH radical production. Spatial distributions showed OH radicals concentrated in sunlit zones, while bleach products showed higher concentrations in the living room due to non-uniform airflow. Analysis of temporal and spatial scales revealed three distinct scales: microscale (<~0.1 m), room scale (~0.1–10 m), and building scale (>~10 m). Short-lived species exhibited sharp spatial gradients, while longer-lived species showed gradients within the room or were well-mixed throughout the building. The study also investigated the spatial and temporal variations of particulate matter (PM) of different sizes, showing significant variations in residence times and deposition patterns.

This study challenges the traditional assumption of well-mixed indoor air by demonstrating the existence of significant spatial and temporal heterogeneity in indoor air pollutants, especially for short-lived and moderately long-lived species. The findings highlight the importance of considering complex chemical reactions, surface interactions, and airflow patterns when assessing human exposure to indoor pollutants. The observed spatial gradients underscore the limitations of single-point measurements and the need for more sophisticated modeling techniques. The results have significant implications for risk assessment and exposure estimation, particularly for reactive species generated by common indoor activities like cleaning. The findings support the need for more advanced modeling approaches that accurately capture these dynamic processes to improve the accuracy of exposure assessments and the development of effective mitigation strategies.

This study demonstrates the existence of heterogeneous distributions of indoor air pollutants, particularly for short-lived and moderately long-lived compounds, challenging the traditional assumption of homogeneous mixing. Spatial and temporal scales are controlled by multiple factors, including chemical reactions, surface interactions, and airflow. The integrated modeling approach effectively captured the dynamic behavior of these pollutants. Future research should focus on improving the characterization of surface interactions and their effects on indoor air quality under varying environmental conditions. A better understanding of these factors will improve assessment of indoor air quality, human exposure to pollutants, and indoor-outdoor chemical transport.

The study focused on a single house during a specific bleach cleaning event. The generalizability of the findings to other houses and cleaning scenarios may be limited. The detailed modeling approaches involved several assumptions and simplifications, which could affect the accuracy of predictions. Further research should investigate the impact of varying building characteristics, ventilation rates, and cleaning methods on the spatial and temporal distribution of indoor pollutants.